Free form fabrication (FFF) and additive manufacturing (AM) are names for a general class of layer manufacturing (LM), in which a three-dimensional (3-D) article is made by the sequential build-up of layers of material. Prior to physically building up the article, the process often begins with creating a computer aided design (CAD) file to represent the image or drawing of a desired article. Using a computer, information about this article image file is extracted, such as by identifying information corresponding to individual layers of the article. Thus, to derive data needed to form an article by LM, conceptually the article is sliced into a large number of thin layers with the contours of each layer being defined by a plurality of line segments or data points connected to form polylines. The layer data may be converted to suitable tool path data, such as data that is manipulated by or in the form of computer numerical control (CNC) codes, such as G-codes, M-codes, or the like. These codes may be utilized to drive a fabrication tool for building an article layer-by-layer.
One or more suitable LM techniques may be utilized for making articles, (e.g., such as by creating one or more device patterns directly on a substrate). The LM technique usually includes a step of selectively depositing material layer by layer, selectively removing material layer by layer, or a combination thereof. Many LM techniques are attractive in that they avoid the need for masks, for pre-existing three-dimensional patterns, and/or expensive tooling.
Historically, LM processes that use electron beams for melting a metal have been generally performed in an open loop fashion, which relies throughout substantially the entirety of the process upon human intervention, and particularly an operator, to adjust operating parameters. For example, an operator typically is obliged to visually observe the LM process throughout the layer by layer buildup, generally external of an LM apparatus and through a viewing port of the LM apparatus. If and when an operator detects a perceived departure from the buildup process, as forecasted, the operator needs to immediately change operating parameters. This approach may pose potential for complications due to the subjectivity of the observations of the operator, due to any delay experienced between an observation and any adjustment in operating parameters, and/or due to improper selection of parameters.
In recent years, there has been a growing need for a reliable system that reduces reliance upon human operators of LM processes and equipment. However, the art has yet to provide an effective solution.
Among the difficulties encountered in attempting to implement closed loop controls for LM techniques, and especially in the area of LM that employs layer by layer build up of articles using molten metal, has been the ability to suitably monitor deposits of metal. This is a particularly acute difficulty when attempting to conduct LM at relatively high output rates. For example, until the present invention, it has been impractical to use camera-based monitoring systems, especially monitoring systems that control metal deposition using overhead imaging of a metal deposit. Optics may be susceptible to vapor build-up that occurs during manufacturing. The amount of data and the rate at which images must be captured for analysis also has faced limitations due to camera hardware. By way of illustration, in achieving rapid imaging, heat may be generated by operation of the associated camera electronics; this may have the effect of corrupting images that are obtained. Overhead positioning of a camera also puts the camera at potential risk of image distortion due to pixel excitation by scattered electrons. Also, suitably robust, commercially practical, and compact designs for use, especially in LM, have been unavailable.
Accordingly, there continues to be a need in the art for an improved system for monitoring layer manufacturing to provide feedback controls for forming a three-dimensional article. More particularly, a system that provides automatic alteration of processing conditions based on information obtained from monitoring the layer manufacturing of the three-dimensional article.
Examples of efforts to provide layer manufacturing of articles and processes include those disclosed in U.S. Pat. Nos. 5,534,314; 5,669,433; 5,736,073; 5,871,805; 5,960,853; 6,401,001; 6,193,923; 6,405,095; 6,459,951; 6,680,456; 7,073,561; 7,168,935; and 7,326,377; and US Patent Application Nos. 20030075836; 20050173380; and 20050288813, all of which are incorporated by reference for all purposes. The possibility of closed loop controls for additive manufacturing of articles by electron beam fabrication processes is identified at col. 12, lines 8-15 in U.S. Pat. No. 7,168,935 (incorporated by reference). In Seufzer and Taminger, “Control of Space-based electron Beam Free Form Fabrication” (accessed at ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20070030308—2007030399.pdf) (incorporated by reference), the authors address a possible approach to a closed-loop control system. See also, Sharma, “On Electron Beam Additive Manufacturing Process for Titanium Alloys” (Abstract for Session on Apr. 27, 2009 Spring 2009 AlChE National Meeting) (incorporated by reference).
U.S. Pat. No. 6,091,444 (incorporated by reference) elaborates on some of the difficulties faced in imaging high temperature melts. The patent illustrates an example of a high temperature melt view camera that includes a water cooled enclosure with a pinhole in it, through which a gas is passed.